1. Field of the Invention
The present invention relates, in general, to a mixed flow pump having a diffuser section with diffuser blades for guiding flow therein.
2. Description of the Related Art
A conventional mixed flow pump, shown in a cross sectional view in FIG. 12, is comprised of a casing 16 housing an impeller 12 rotating about an axis of a rotation shaft 10, and a stationary diffuser section 14 disposed downstream of the impeller 12. The flow passage P in the diffuser section 14 is formed as a three-dimensionally curved space in a ring-shaped space formed between the casing 16 and a hub 18, separated by diffuser blades 20. A fluid medium taken through a pump inlet 22 is given kinetic energy by the rotating impeller 12, and is reduced of its circumferential velocity as the fluid enters into the stationary diffuser section 14, and the kinetic energy at the impeller exit is recovered as a static pressure in the pumping system.
The shape of the flow passage P in the diffuser section 14 is defined according to the shape of the meridional (axisymmetrical) surfaces of the hub 18 and the casing 16 and the geometrical shape of the diffuser blades 20. Of these three, the shape of the blades is determined by choosing a distribution pattern of blade angle xcex2 which is an angle between a direction M tangential to a center line of the blade on the axisymmetrical surface of the hub 18 or the casing 16 at any given point along the blade length and the tangent L in the circumferential direction at that point, as illustrated in FIG. 13A.
The blade angle xcex2 is given by an equation relating the meridional distance m (defined by the distance along the line of intersection of a plane containing the rotation axis of the impeller 12 and the axisymmetrical surface) and a circumferential coordinate xcex8 and a radial coordinate r for the blade center line as follows (refer to FIG. 13C):
xe2x80x83tan xcex2=dm/d(rxcex8)xe2x80x83xe2x80x83(1)
The blade angle xcex2 of the diffuser blade 20 at the entrance-side of the diffuser section 14 is chosen to coincide with the direction of the stream flow at the exit of the impeller 12, and the blade angle xcex2 of the diffuser blade 20 at the exit-side of the diffuser section 14 is chosen so that the exiting flow is produced primarily in the axial direction after being eliminated of the circumferential velocity component of the flow. In the flow passage that lies between the entry and exit regions of the diffuser section 14, it is a general practice in the conventional design technology to adopt a smooth transition of blade angles resulting in that, as shown in FIG. 14A, the blade angle distribution pattern is similar along the hub surface and along the casing surface. In the illustration shown in FIG. 14A, the non-dimensional distance m* is defined by normalizing the meridional distance m by the distance l from the leading edge to the trailing edge of a blade along either the hub surface or, the casing surface. FIG. 15 shows the blade angle distribution pattern of the blade angle difference xcex94xcex2 between the hub blade angle and the casing blade angle in a conventional diffuser section operating in a specific speed range between 280xcx9c700 (m, m3/min, rpm) with respect to the non-dimensional distance m*. It can be seen that, in either case, the absolute value of the blade angle difference |xcex94xcex2| in the distribution pattern is less than 10 degrees, indicating that the blade angle distribution patterns at the hub surface and at the casing surface of a blade are substantially similar along any blade.
However, actual flow fields in the diffuser section in an operating pump are composed of complex three-dimensional flow patterns, and the frictional effects along the walls on the flow passage produce low-energy fluids which tend to accumulate at the corner regions of the suction surface and the hub surface due to the secondary flows action. In the conventional designs, a smooth merging of flow passage is produced by choosing the blade angle distribution as described above. However, because the three-dimensional flow fields are not taken into consideration, it has been difficult to prevent a large-scale flow separation from being generated at the corner or blade root regions where the hub surface meets with the suction surface of the blade.
FIG. 16 is a schematic plan view of secondary flows generated on the suction surface of the blade, while FIG. 17 is a schematic plan view of the secondary flow patterns generated on the hub surface in the conventional technology. The low-energy fluids accumulated at the blade root regions of the diffuser section do not have sufficient kinetic energy to overcome the pressure rise in the diffuser section, and as a result, flow separation and reverse flow occur in these blade root regions as illustrated in FIG. 17.
In the following, the problems encountered in the conventional diffuser section designs will be explained in further detail with reference to a three-dimensional viscous flow analysis. FIG. 18A shows contour lines of the static pressure distribution diagram on the suction surface of the blade, and FIG. 18B shows the contour lines of the total pressure distribution diagram in the flow passage section at a non-dimensional distance m*=0.59, and FIGS. 19A and 19B show the predicted velocity vectors close to the suction surface and the hub surface.
As shown in FIG. 18A, in the conventional diffuser section, the contour lines in the entry section of the suction surface (region A) are roughly parallel to the flow passage P. The flow streams having lost their kinetic-energy through the frictional effects along the blade wall are not able to resist the adverse pressure gradient, and generates secondary flows along the contour lines in the static pressure distribution diagram, as shown in FIG. 19A.
Because the flow velocity is high in the diffuser entry section, especially near the suction surface, a large friction loss is generated on the blade walls, and the low-energy fluids are drawn by the secondary flows on the suction surface and accumulate in the corner regions (region B) formed between the downstream hub section and the suction surface.
As can be understood from the dense distribution of the contour lines shown in FIG. 18A, the adverse pressure gradient is high at the corner region B, thus generating a large-scale flow separation as illustrated in FIGS. 19A and B, thereby causing a significant loss in the pumping efficiency. This situation becomes more acute, especially when the pump is made compact, because the loading on the blade increases and leads to an increase in the adverse pressure gradient, so the pump becomes even more sensitive to the separation phenomenon. These are some of the basic reasons that have prevented the conventional technology from making compact and high efficiency pumps.
It is an object of the present invention to provide a highly efficient mixed flow pump by optimizing secondary flows in the diffuser section so as to prevent flow separation which is likely to occur in the corner region of the flow passage of the diffuser section.
The object has been achieved in a mixed flow pump comprising a casing having an axis and defining an impeller section and a diffuser section disposed downstream of the impeller section. The impeller section comprises an impeller rotating about the axis. The diffuser section has a hub and stationary diffuser blades, wherein the diffuser blades are formed so that an angular difference, between a hub blade angle and a casing blade angle, is chosen to conform to a specific distribution pattern along a flow passage of the diffuser section. Accordingly, by choosing an appropriate design of the blade angle of the diffuser blades, a suitable pressure distribution pattern along the flow passage in the diffuser section is obtained by optimizing secondary flows.
In the mixed flow pump presented, the blade angle may be defined in terms of an angle between a circumferential tangent line at a point on the blade surface at a level of hub surface or casing surface and a tangent line of a center line of a cross section of the blade along the hub surface or casing surface, and the specific distribution pattern is such that a hub blade angle is greater than a casing blade angle in a wide range of the flow passage. Accordingly, the pressure rise along the hub surface is completed before the pressure rise along the casing surface so that the flow speed reduction along the hub surface is completed before the flow speed reduction on the casing side, thereby enabling the static pressure recovery on the hub side to supercede the recovery on the casing side of the pump.